Did God commit adultery impregnating Mary since she was considered married to Joseph?
God did not commit adultery by impregnating Mary. First, such a premise is both ridiculous and reveals an unnatural and insincere reading of the biblical text. The Bible describes the conception of Jesus in Mary in very different terms:
When Mary asked how she could give birth as a virgin, the angel replied that the power of God would overshadow her, meaning the Creator would create a child inside her, much as He created the first man in the Garden. Just as Adam had no human father, nor would Christ have a human father but would be created out of the womb of Mary by God.
The circumstances don’t even fit the definition of adultery. Adultery is defined as an act of consensual sexual intercourse between a man and woman when one or both are married to someone else. In the case of the conception of Mary by the Holy Spirit, none of these parameters apply. The Holy Spirit is not a human being, so He cannot commit adultery. Secondly, the Holy Spirit didn’t impregnate Mary through sexual intercourse, so no sexual act took place (Mary remained a virgin).